Copy/Paste pose operator not working
On your second armature &quot;Armature.001&quot;, the mode &quot;Rest Position&quot; is enabled. That prevents you from modifying the pose.
Select &quot;Armature.001&quot;, go to &quot;Object Data&quot; properties, then select &quot;Pose position&quot; instead of &quot;Rest position&quot;. The Copy/Paste operation should now work as intended.